Heat exchanger provided with a thermal barrier

ABSTRACT

Heat exchanger for producing steam mainly formed by a casing in which a bundle of steam tubes is provided, attached between tube sheets and inside of which casing liquid metal such as sodium flows around these tubes as a heat-transferring medium, the transition area from tubes to tube sheet in this casing being provided with a thermal barrier to reduce the heat transfer from the sodium to the tube sheet and to the tubes near their attachment to the tube sheet, this thermal barrier being formed by a number of tubular spaces enclosing one or more tubes of the tube bundle along a part of their length(s) which tubular spaces are formed by a number of baffles together forming a grid and are closed on the one side by the tube sheet and on the other side are in open connection with the inner space of the rest of the heat exchanger casing.

3,029,796 4/1962 Simmons etal Inventor Appl. No.

Filed Patented Assignee Priority Kornelis Lievense Spijkenisse, Netherlands May 21, 1969 Dec. 28, 1971 Nederlandse Organisatie voor Toegepast- Natuurwetenschappelijk Onderzoek ten behoeve van Nijverheid, Handel en Verkeer May 30, 1968 Netherlands HEAT EXCHANGER PROVIDED WITH A 136,158 MF, 158 F; 122/34 References Cited UNITED STATES PATENTS 2,825,463 3/195'8 Thomas .L 210/176 3,132,691 5/1964 Esleeck 165/178 X 3,140,792 7/1964 Harris 165/134 X FOREIGN PATENTS 1,205,121 5/1959 Germany 165/158 Primary Examiner- Frederick L. Matteson Assistant Examiner-Theophil W. Streule Attorney-Hammond & Littell ABSTRACT: Heat exchanger for producing steam mainly formed by a casing in which a bundle of steam tubes is provided, attached between tube sheets and inside of which casing liquid metal such as sodium flows around these tubes as a heat-transferring medium, the transition area from tubes to tube sheet in this casing being provided with a thermal barrier to reduce the heat transfer from the sodium to the tube sheet and to the tubes near their attachment to the tube sheet, this thermal barrier being formed by a number of tubular spaces enclosing one or more tubes of the tube bundle along a part of their length(s) which tubular spaces are formed by a number of baffles together forming a grid and are closed on the one side by the tube sheet and on the other side are in open connection with the inner space of the rest of the heat exchanger casing.

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K ORNE LIS LIEVENSE ATTORNEYS HEAT EXCHANGER'PROVIDED WITH A THERMAL BARRIER The invention relates to a heat exchanger in which steam is produced and which operates with liquid metal, as for example sodium as heat-transferring medium, mainlyformed by a casing, inside of which abundle of watertubes or steam tubes is provided attached between tube sheets, and inside of which casing the heat-transferring-medium flows around these tubes.

In such heat exchangers a steep temperature. gradient is formed at the sodium side of the tube sheets and at the joints between tubes and tube sheet.

For on the one hand this tube sheet and tubes are highly heated locally and on the other'hand strongly cooled by the water or steam flowing through the holes of the tube sheet and through the tubes; this results in thermal stresses. Locally in the sodium large temperature variations will occur, considerable nonstationary stresses being caused, too.

The thermal stresses arisen from stationary and nonstationary temperature gradients mainly occur in the area of the tube sheet where the tubes are connected with the tube sheet.

Due to the locally higher temperature of the tube sheet arisen from the contact of the hot sodium: with the tube sheet the material strength locally will be adversely affected.

In order to. prevent the above extra strains occurring in. a heat exchanger operating with liquid sodium, it is known from the literature on this subject, to provide means in the transition area from tubes to tube sheet to reduce the heat. transfer from the sodium to the tube sheet and to the joints between tubes and tube sheet.

For this purpose ina known vertical heatexchanger over the sodium column between the sodium and the tube-to-tubesheet joints a layer of inert gas is created, which serves as a gas shield and forms a kind of thermal barrier.

Such a gas shield has its drawbackscan for example only be embodied in a heat exchanger of the vertical type, which further necessitates a construction with one or more bent tube bundles so that on either side of these tube bundles such a gas shield can be applied.

From the ATomic Energy Commission Report No. 3,014 OR.NL. part I (heat exchanger for M.S.R.E.) Fig. 1, 4, pp. 8-10 inclusive a heat exchanger is known in which a similar kind of thermal barrier is formedby application of a battle arranged transversely to the tube bundle at some distance of the tube sheet, themedium being stationary between this transverse baffle and the tube sheet, causing likewise a kind of thermal barrier grid to be formed for reduction of the heat transfer to the tube sheet fromthe heat-transferring medium flowing through the heat-exchanger.

A large drawback of sucha transverse baffle is, when sodium is applied as heat-transferring-medium in a steam generator that if a leakage should arise in a tube-to-tube-sheet joint, the pressure of a sodium-water reaction. created by this will push aside the transverse baffle in'axial direction, causing a great damage to arise because the transverse baffle-will injure the tubes.

It is the object of the invention to provide a heat exchanger that does not have the above drawbacks and to provide means for reduction of theheat'transfer to the vital joints between are in open connection with the inner space of the heat.

exchanger.

Because these tubular spaces are in'open connection withthe inner space of the heat exchanger, and so with the sodium, a part of the sodium fed into the heat exchanger will penetrate into these tubular spaces and come to a standstill against th tube sheet.

In this way at the ends of the tubes and against the tube sheet a layer is formed of substantially stagnate sodium, built up of relatively narrow columns of liquid lying next to each other against the tube sheet and around the tubes.

, tubes and tube sheet, thus avoiding the penetration of the tem- The sodium in the tubular spaces will adopt the temperature of the steam tubes passing through them.

As a result of the application of these tubular spaces no steep temperature gradient can occur in the tube sheet and the tube-to-tube-sheet joint.

Locally the strength of the material of tubes and tube sheet on the spot will not be adversely affected either.

Due to the slight velocity of the sodium and a substantially not changing temperature in the sodium layer formed thermal shocks from the sodium will be considerably reduced or prevented.

lf, against expectation, at the tube-to-tube-sheet joints any rises in pressure should nevertheless occur, then the sodium can escape through the open connections of the tubular spaces with the remaining part of the heat exchanger.

By preference the tubular spaces will be composed of a number of baffles, together forming a grid, and at one side abutting to the tube sheet.

This grid can for example be composed of two rows, provided at a certain angle, of several bafl'les provided parallel to each other, these baffles being provided with notches in such a way that a number of tubular spaces (sleeves) are formed, enclosing the tubes.

Further, the achievement of a stagnate layer of sodium can be promoted by placing the baffles in such a way in respect to each other that each sleeve formed by these baffles enclose an individual water tube or steam tube.

The invention will hereinafter further be elucidated and explained with reference to a drawing, in which an embodiment of the most important parts of the heat exchanger according to the invention is shown.

FIG. lshows a cross section of part of the tube bundle of a heat exchanger in which the positioning of the grid construction composed of tubular channels against the tube sheet is shown.

FIG. 2 gives a cross section accordiirgto the line ll-ll in FIG. 1 in the direction of the tube sheet, the arrangement of the baffles being shown.

Finally FIG. 3 shows in what way this grid construction is situated with respect to the remaining parts of the heat exchanger.

In F IG. 1, l are the water tubes or steam tubes, which are attached to tube sheet 2, the tubes corresponding with holes 3 provided in the tube sheet for the connection of the steam tubes with header 4.

The grid construction built up from baffles 5 is provided within a short open cylinder 6, with which the grid construction is attached to the tube sheet 2.

For that purpose open cylinder 6 is provided along its periphery with a flange-shaped ring 7 provided with holes, with which the cylinder is attached to the tube sheet by means of tapped bolts 8 and nuts 9.

A large number of tubular channels 10, which enclose the steam tubes locally, are formed by two rows, provided at an angle, of bafiles 5, mutually parallel (see FIG. 2).

These tubular channels are closed at tube sheet side 11 and on the other side 12 they form an openconnection with the remaining inner space of the heat exchanger in which the sodium flows around the steam tubes.

Part of thesodium-supplied via sodium supply duct 13 (see FIG. 3) will penetrate via annular space 16 formed between casing 14 and cylindrical flow baffle 15 into tubular channels 10, in which the flow speed of the sodium will greatly diminish, and the sodium coming to a standstill against the wall-of the tube sheet.

Thus a layer of stagnate sodium is obtained, adjacent to the tube sheet and built up from relatively narrow columns of liquid sodium lying next to each other and around the tubes at their connections to the tube sheet.

Locally the sodium will adopt the temperature of the steam tubes lying in the tubular channels.

As a result no steep temperature gradient will occur at the tubes and the tube-to-tube-sheet joint.

The strength of the material of tubes and tube sheet will not be affected adversely. 1

The occurrence of thermal shocks from the sodium with as a result the onset of thermal stresses in the tube-to-tube-sheet joints can substantially be prevented by using the above construction.

Should unexpectedly pressure rises nevertheless occur in the sodium at the vital joints between the tubes and tube sheet, then the sodium can escape easily via the open connections of the tubular spaces with the rest of the heat exchanger.

I claim: 7

l. A heat exchanger for producing steam and operating with liquid metal as a heat-transferring medium comprising a plurality of water or steam tubes attached between two tube sheets, a casing enclosing said plurality of tubes and in which liquid metal flows around said tubes and a thermal barrier means in the transition area from tubes to tube sheets to reduce heat transfer to the tube sheets and the tubes near their attachment to the tube sheets, said thermal barrier means comprising a plurality of tubular spaces with their walls parallel to the tube walls and enclosing one or more of the tubes along a portion of their length, which spaces are closed on the one side by the tube sheet and on the other side are in open connection with the inner space of the heat exchanger.

2. A heat exchanger according to claim 1, wherein the tubular spaces are formed by a number of baffles, together forming a grid, and at one side abutting to the tube sheet.

3. A heat exchanger according to claim 1, wherein each tubular space encloses an individual tube. 

1. A heat exchanger for producing steam and operating with liquid metal as a heat-transferring medium comprising a plurality of water or steam tubes attached between two tube sheets, a casing enclosing said plurality of tubes and in which liquid metal flows around said tubes and a thermal barrier means in the transition area from tubes to tube sheets to reduce heat transfer to the tube sheets and the tubes near their attachment to the tube sheets, said thermal barrier means comprising a plurality of tubular spaces with their walls parallel to the tube walls and enclosing one or more of the tubes along a portion of their length, which spaces are closed on the one side by the tube sheet and on the other side are in open connection with the inner space of the heat exchanger.
 2. A heat exchanger according to claim 1, wherein the tubular spaces are formed by a number of baffles, together forming a grid, and at one side abutting to the tube sheet.
 3. A heat exchanger according to claim 1, wherein each tubular space encloses an individual tube. 